The phenomenal growth in the demand for wireless communications has put persistent pressure on wireless network operators to improve the capacity of their communication networks. To improve the spectral efficiency of these networks, scarce radio resources have to be reused aggressively in neighboring cells. As a result, inter-cell interference has become a main source of signal disturbance, limiting not only the service quality to users at the cell edges, but also the overall system throughput.
A multi-antenna wireless communication system, such as one employing Coordinated Multipoint (CoMP) transmission and/or reception, can mitigate inter-cell interference. In such a system, multiple antennas are deployed across a plurality of geographically contiguous cells—referred to as sub-cells—and connected to a central controller, e.g., via a fast backhaul. This architecture enables the central controller to jointly process downlink signals transmitted from and/or uplink signals received at the multiple antennas, in order to mitigate inter-cell interference.
To jointly process downlink signals transmitted from the multiple antennas, in particular, the central controller must have available to it information characterizing the associated downlink channel responses. If the system employs frequency-division duplexing (FDD), the served mobile terminals must measure these downlink channel responses and explicitly feed them back to the central controller. If the system instead employs time-division duplexing (TDD), the downlink channel responses can advantageously be estimated from uplink signals received at the multiple antennas, based on the assumption that for TDD the downlink channel can be inferred from the uplink channel.
This assumption of reciprocity between the downlink and uplink channel, however, may sometimes be inaccurate in a multi-antenna system. For instance, each of the multiple antennas is connected to a corresponding transceiver that comprises a transmit-receive radio frequency (RF) chain. These transmit-receive RF chains may have different frequency responses, due for example to differences in the transfer characteristics of the components (e.g., analog filters, power amplifiers, etc.) making up those RF chains. If the RF chains of the multiple antennas do not have identical frequency responses, the assumption of reciprocity no longer proves accurate, which in turn prevents advantageous estimation of the downlink channel responses from uplink signals. Accordingly, the RF chains must be initially, and perhaps periodically, calibrated with one another to account for differences in their frequency responses.
Known approaches to calibrating the RF chains of antennas in a multi-antenna system rely on mobile terminals to assist with the calibration process. These approaches must identify at least one mobile terminal that is sufficiently stationary (i.e., has a relatively low Doppler spread) and that has a sufficient quality of service (i.e., a strong channel quality indicator, CQI) to adequately measure and feed back the downlink channel responses. If such a suitable mobile terminal can be identified, calibration factors can be computed based on the provided feedback and then applied to the RF chains. Of course, a suitable mobile terminal may not always be available, meaning that these approaches are inherently undependable. Further, by requiring feedback of the downlink channel responses, the approaches necessarily diminish the very advantage sought by calibrating the RF chains in the first place. Still further, these and other approaches limit the scalability of the calibration process to a relatively small number of antennas.